A heat shrinkable, thermoplastic, multi-layer packaging film which has enjoyed considerable commercial success for making bags having low gas transmission is described in U.S. Pat. No. 3,741,253 which issued on june 26, 1973 to H. J. Brax et al. One preferred film in the Brax et al patent may be produced by extruding a first or substrate layer of ethylene-vinyl acetate copolymer as an unstretched tube and then flatting the tube and moving it in a series of passes through an electron beam to cross-link the molecules of the copolymer. Next, the film is inflated, but not substantially stretched, and passed through an annular coating die where it receives a second layer which comprises a copolymer of vinylidene chloride. This two layer tube then passes through another annular die in which it receives a coating of ethylene-vinyl acetate copolymer. The tube which now has a wall of three polymeric layers and is cooled, collapsed, and fed through a hot water bath where the tube is sufficiently softened so that it can be inflated into a bubble which is trapped between two spaced apart pairs of pinch rollers. The inflation causes the tubing to be thinned as it is stretched into film, and, as the bubble leaves the hot water bath it cools quickly in air and becomes oriented. After the bubble is collapsed, the film is wound up as flattened, seamless, tubular film to be later used to make bags, e.g. both (1) end-seal bags which are typically made by transversely heat sealing across the width of flattened tubing followed by severing the tubing so that the transverse seal forms the bottom of a bag, and (2) side-seal bags in which the transverse seals form the sides and one edge of the tubing forms the bottom so the bags can readily be made from the tubing.
In making bags as described above in a rapid commercial operation by pressing the flattened tubing walls together with heated seal bars, the dwell time of the seal bars should be as short as possible yet sufficiently long to fuse and bond the inside surface layer of the tube to itself in a smooth, continuous, strong seal. The importance of a strong reliable seal is readily understood when the use and function of the bag is described. This type of bag is primarily used to package a large variety of fresh and processed meats by placing the meat in the bag, evacuating the bag, gathering and applying a metal clip around the gathered mouth of the bag to hermetically seal it, and then immersing the bag in a hot water bath at approximately the same temperature at which the film was stretched-oriented. This temperature in the past has typically run from 160.degree. to 205.degree. F. Hot water immersion is one of the quickest and most economical means of transferring sufficient heat to the film to shrink it uniformly. However, one of the objects of the present invention is to extend the time at which a bag can maintain its integrity at high temperatures beyond the time normally required for shrinking so that the product in the bag can become pasteurized.
A problem which is associated with the manufacture of side seal bags on certain commercially available machines is that at rapid speeds the seals do not have time to "set-up" and the tension when transporting the tubing can cause the seals to pull apart. For example, when side-seal bags are made from flattened tubing a transverse double seal with a perforation line therebetween is made across the tubing. The double seal comprises two parallel, spaced apart seals and between the two seals and parallel to them a series or line of perforations is cut. One longitudinal side of the flattened tubing is trimmed off either before or after the double seal is applied to form the top or mouth of the bag. The distance between perforation lines is the overall width of the bag.
Intermittent motion is necessary as bags are moved to and from the seal station as there must be a time when the tubing is stopped while the transverse seals are made. To move the bags away from the seal station a bag can be threaded around a reciprocating roller to pull the tubing for the next bag into place as is the case with the side seal bag machine of Gloucester Engineering Co. of Gloucester, Mass. At a high bag making rate if the side seals are not strong, the force due to the rapid acceleration of the leading bag from a dead stop to advance the next bag into the seal station will cause the newly formed side seals to be pulled apart as the seals are still warm. Prior practice was to simply reduce machine speed and bag making rate to allow the seal to cool. However, it is an object of this invention to provide a film which will have strong seals at high bag making rates.
In addition to end and side seal bags, other receptacles such as pouches and casings can be made from thermoplastic materials. A pouch can be formed by placing a product between two separate sheets of film and then heat sealing the sheets together generally around the periphery of the product to enclose it. A casing can be formed from either seamless tubing by sealing or closing one end of the tube in a gathered closure or by forming a tube from a sheet by overlapping or butt heat sealing of opposed edges of the sheet together to form a tube afterwhich a gathered closure is used to close one end of the tube. A satisfactory and commonly used gathered closure means is a metal clip. After filling the casing the open end usually will be closed also with a clip. Accordingly, an object of the present invention is to provide a film from which pouches and casings can be made which will withstand pasteurization temperatures.
Another object of this invention is to provide a film with superior high temperature seal strength and which also will produce a bag having gas transmission characteristics at least approximately equal to prior art films. Many food products require pasteurization if they are hermetically packaged. The pasteurization is necessary to destroy harmful microbes which grow in the absence of air. In some countries, for certain products, the requirement of the health laws and regulations will be to hold the food product in the evacuated, hermetically closed bag at a temperature of at least 93.degree. C. (200.degree. F.) for 3 minutes. In other countries the requirement may be to hold the product at 82.degree. C. (180.degree. F.) temperature for 30 minutes. Thus, it is yet another object of this invention to provide a bag which will maintain seal integrity for extended time periods at temperatures up to the boiling point of water.
Still another object is to provide in one film the high temperature strength of propylene polymers and the hot water shrink capability of ethylene polymers. Propylene homopolymers and copolymers tend to have high melting points, in excess of 300.degree. F., whereas ethylene polymers and copolymers tend to melt near and below the boiling point of water, generally below 220.degree. F.
The foregoing objects are achieved by the present invention which is summarized in the paragraphs below.